Kubernetes Classroom Series – 08/Jul/2021

Singleton Service

• Overview: Singleton Pattern ensures only one instance of an application is active at a time and is highly available
• Problem:
  • In some cases only one instance of the service is allowed to run
• Solution:
  • Out of Application Locking:
    • Stateful set or replicat set with replicas = 1
  • In-Application Locking
    • In a distribute environment one way to control the service instance count is through a distributed lock
    • We can implement leader election using kubernetes api’s. For example apache camel has a K8s connector that also provides leader election and Singleton capabilities
    • This connector access the etcd api directly and k8s api to use Config Maps to acquire a distributed lock Refer Here
    • Refer Here for sample leader election in k8s.

DNS For Services and Pods

• Refer Here for the manifests used
• Lets create the test pod
• Make a note of service names and ip addresses of pods
• Now login into test pod

kubectl exec -it test -- /bin/sh

• Look into resolve.conf

• What objects in k8s gets DNS records

  • Services
  • Pods

• Refer Here for the K8s DNS Based Service Discovery

• DNS A record Refer Here

• DNS CNAME Refer Here

• DNS SRV Refer Here

• K8s Services:

  • A/AAAA records
    • Normal Services are assigned a DNS A record depending on the IP family of the service for the name in the form of my-svc.my-namespace.svc.cluster-domain.example
  • SRV records:
    • SRV records are created for each named ports that are part of normal or Headless Services. For each name port the SRV record would have the form _my-port-name._my-port-protocol.my-svc.my-namespace.svc.cluster.local

• K8S Pods

  • A/AAAA records:
    • Pod has the following dns resolution

    pod-ip-address.my-namespace.pod.cluster.local
    

• In kubernetes in addition to DNS records k8s create environmental variables with service information